Tunis: Four years after its revolution sparked the Arab Spring, Tunisia`s presidential campaign neared its climax Friday with incumbent Moncef Marzouki facing 88-year-old favourite Beji Caid Essebsi in a runoff.

Ahead of Sunday`s landmark second-round vote, jihadists issued a videotaped threat against the North African country`s political establishment.

Essebsi held a last rally on Bourguiba Avenue in Tunis, a focal point of the 2011 revolution that toppled president Zine El Abidine Ben Ali, while Marzouki spoke to supporters just outside the capital.

Capping off four years of a sometimes chaotic transition, the vote is the first time Tunisians are freely electing their president since independence from France in 1956.

Amid tight security and the closure of main border posts with strife-torn neighbour Libya, almost 5.3 million Tunisians are eligible to vote.

Citizens living abroad began voting on Friday.

The November 23 first round saw Essebsi, who heads the anti-Islamist Nidaa Tounes party, take 39 percent of the vote.

Marzouki, a 69-year-old former rights activist installed by parliament two months after December 2011 polls with the backing of the moderate Islamist party Ennahda, took 33 percent.

Nidaa Tounes won an October parliamentary election and Essebsi has emerged as the clear favourite to be the next president.

But the campaign has been marked by mudslinging, with Essebsi refusing to take part in a debate with Marzouki, claiming his opponent is an "extremist".

Essebsi insists that Marzouki represents the Islamists, and on Friday he urged supporters to vote for him and "a better future for Tunisia", saying Islamists had "ruined" the country. Throughout the campaign, Marzouki has accused Essebsi, who served as a senior official in previous Tunisian regimes, of wanting to restore the old guard deposed in the revolution.

He has even suggested that Essebsi`s camp was preparing to "win through fraud", drawing a sharp rebuke from Tunisia`s electoral commission.

The final result is expected to be announced between December 22 and 24.

In an Internet video posted on Wednesday night, jihadists who joined the Islamic State group claimed the 2013 murder of two secular politicians that plunged Tunisia into crisis, warning of more killings of politicians and security forces.